Why: Because. :-)

Does it matter: Possibly, yes. Certain players can have serious problems during layer changes, and it's nice to know (if you have those problems) whether it's related to a layer change or something else.

Is it useful information: Yes. The placement of a layer change, when improperly done, can create a noticable "blip" in the movie. For tech-heads, the first thought is "what happened; did something break". It breaks concentration on the film. Even HOW the layer change is done can determine whether it's invisible (Lost in Space), nearly invisible (Shakespeare in Love) or dang noticable (Godzilla). It's as much a part of the technical mastering of a particular disc as the video or audio transfer quality.

Not to mention that such knowledge makes GREAT trivia questions at parties when you're trying to impress the girls.